1) Field of the Invention
The present invention relates to organic sulphur compounds which can be employed as antiwear and extreme-pressure additives to lubricants, to the synthesis of
these compounds and to lubricating compositions containing the said compounds.
2) Description of the Related Art
Extreme-pressure additives are employed chiefly in oils for industrial or motor vehicle gear drives or for specific lubricants for machining metals. These compounds, which generally contain sulphur, chlorine, phosphorus or nitrogen, act by forming a surface layer which prevents the formation of local microwelds. These microwelds are produced by high-amplitude heat effects which appear between two surfaces moving relative to each other under high stresses.
The extreme-pressure additives currently employed can be classified into four groups.
The first group concerns the products resulting from the reaction of alcohols, phenols, olefins and/or hydrocarbon compounds containing sulphur, chlorine and nitrogen atoms with phosphorus derivatives such as phosphorus pentasulphide. These products are described, for example, in U.S. Pat. No. 4,058,463. The extreme-pressure effect of these sulphur-containing and phosphorus-containing additives is generally insufficient, but they have advantageous additional properties. In particular, they are antioxidants and are noncorrosive.
The second group concerns the products formed by reaction of mono- or polyunsaturated olefins or of aromatic compounds which are optionally substituted by alkyl chains, or by hydrocarbon chains containing heteroatoms such as sulphur, chlorine, phosphorus or nitrogen, or else fatty acid esters with chlorine-containing sulphur compounds (SCl.sub.2, S.sub.2 Cl.sub.2, R-S-Cl).
The product obtained is in some cases treated with an alkali metal hydroxide, a sodium mercaptide or a sodium polysulphide to reduce the residual chlorine content. These products are described, for example, in U.S. Pat. Nos. 4,204,269, 4,198,305, 4,097,387, 3,925,414 and 3,844,964 and French Pat. No. 2,404,042.
French Pat. No. 2,588,881 recommends adding an alkenyl chloride before the treatment with an alkali metal polysulphide to improve the solubility of the final products in the oil.
French Pat. No. 2,605,328 recommends the subsequent treatment with an organic polysulphide as a replacement for sodium polysulphide to improve the solubility of the final products in mineral and synthetic oils.
The products obtained according to these processes have medium to high sulphur contents (up to 50%), endowing them with excellent extreme-pressure properties, but they are frequently corrosive. One of the major disadvantages in the synthesis of these products concerns the final washing stages, which increase the cost of the process. These products generally contain a little residual chlorine, and this may limit their use in some applications.
The third group comprises the products resulting from the reaction of mono- or polyunsaturated olefins, unsaturated fatty acid esters or of hydrocarbon compounds containing heteroatoms such as sulphur, chlorine, phosphorus or nitrogen with elemental sulphur or a mixture of elemental sulphur and of hydrogen sulphide, in the presence or in the absence of catalyst. There are, for example, known processes using the reaction of isobutylene or of C.sub.3 -C.sub.8 olefins with sulphur, as mentioned in U.S. Pat. Nos. 4,119,850, 4,119,545 and 3,899,475.
The fourth group comprises the products of addition of inorganic sulphur derivatives to epoxides.
German Pat. No. DE 36 04,793 claims the preparation of antiwear and extreme-pressure additives by condensation of epoxides which are preferably obtained by epoxidation by alpha-olefins containing 6 to 20 carbon atoms with a sodium polysulphide such as Na.sub.2 S.sub.2 or Na.sub.2 S. The products generally contain from 15 to 30% of sulphur and are noncorrosive towards noble metals.
U.S. Pat. No. 3,064,056 recommends the preparation of bis(beta-hydroxyalkyl) di- or trisulphides by reaction of an epoxide with elemental sulphur in the presence of basic catalyst.
All these products have a symmetrical structure, and this considerably limits the number of possible variants.